Described herein is a heating and/or air-conditioning device for a motor vehicle, the device including an inlet duct for the supply of fresh air, an air transmission branch, and an air heating branch, both supplied by the inlet duct and communicating with each other via a mixing zone. A butterfly flap is mounted pivotably around a rotation axis located at the mixing zone. The butterfly flap comprises a first wing associated with a first branch and a second wing associated with a second branch, as well as a wing extension elastically hinged to the first wing according to a hinge axis parallel to the rotation axis of the butterfly flap. Within the first branch are arranged abutment formations suitable to be engaged by the wing extension and to induce, during a rotation of the butterfly flap, a rotation of the wing extension with respect to the first wing.